1. Field of the Invention
The present invention relates to a method for prolonging the life span of a light source generating apparatus. More particularly, the present invention relates to a method for prolonging the life span of a planar light source generating apparatus.
2. Description of the Related Art
The principle of light emission in a field emission display device is based on the occurrence of electron emission at a tip of material in a vacuum environment due to there existing a strong electrical field. These field-emitted electrons leaving a cathode plate accelerate toward a positively charged anode plate and ultimately bombard with fluorescent material disposed thereon to produce light. Conventionally, the cathode plate serves as a source for producing the field electrons and the anode plate serves as a light source. FIG. 1 is a diagram showing a conventional field emission apparatus. As shown in FIG. 1, the electrons emitted from the cathode plate 10 bombard the fluorescent layer 201 disposed on the anode plate 20 to produce light. The cathode plate 10 includes a glass substrate 102 and a gate and emitting layer 101 disposed on the glass substrate 102. FIG. 2 is a top view showing a conventional cathode, a gate and an emitting layer 101, which comprises a plurality of stripe gates 101a and a plurality of stripe cathodes 101b disposed alternately. Furthermore, a plurality of emitting layers 101c is formed on the stripe cathodes 101b. 
The anode plate 20 comprises a glass substrate 203, a conductive reflection layer 202 and a fluorescent layer 201. Furthermore, a heat sink 30 is disposed on the glass substrate 203. The fluorescent layer 201 is fabricated using a fluorescent powder capable of generating the three primary colors, i.e. red, blue and green,. for producing white light or simply fabricated using a white fluorescent powder. The electron emission layer 101c is fabricated using a material with a lower work function, for example, molybdenum (Mo), titanium carbide (TiC), tungsten (W), silicon (Si) or carbon nanotube. Thus, the material layer can be used as an emission source for the electron emission layer. The electrons emitted from the emitting layer disposed on the cathode plate 10 bombard against the fluorescent layer 201 disposed on the anode plate 20 and then produce a mixture of red, blue and green light (that is, white light is thus generated) or directly produce white light if the white fluorescent powder is used. However, the conductive reflection layer 202 disposed on the anode plate 20 reflects the white light. The reflected white light may penetrate through the cathode plate 10 and exit from another surface 10a of the cathode plate 10. Thus, if the field emission display device is used as a back light source, the display device is so disposed closely to the cathode plate, in which the surface of the display device facing the cathode plate 10a is used as a light-receiving surface.
As the reflected light needs to penetrate the cathode plate, an electrode layer and a gate layer of the cathode plate are designed in such a way that they are simultaneously formed at a same layer during a same fabricating step. Furthermore, when the field emission display serves as the back light source for other devices, it is able to generate a planar light source with more uniformly-distributed brightness than other light source, such as, a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED). The electrode and the gate of the cathode plate are electrically connected to an AC voltage and a ground respectively so that the electrons emitted from the emitting layer disposed on the cathode bombard against the fluorescent layer 201. However, for the conventional field emission device whose gate is driven by an AC voltage, the emitting layer disposed on the cathodes is unable to produce enough electrons for generating light with sufficient brightness after being used a period of time.
Therefore, for the manufacturers of the display device, there exists an important issue of how to prolong the life span of the planar light source generating apparatus.